(new:minor) Added node lookup method based on bisection and made it as the default method
(change:major) Completely rewritten and also added the 'generalized natural spline'
(new:major) New interpolation methods: constant (step-like) interpolation and nearest-neighbor interpolation
(change:minor) Updated to include constant and nearest-neighbor interpolation methods.
(change:minor) Updated tests according to last changes to cubic spline interpolation